The objective of this project is to determine the role that hormones, in particular estrogen and progesterone, play in the growth of mammary tumors. We propose to test the hypothesis that the role of estrogen in breast cancer is to act not as a mitogen, but rather to induce in the cells properties essential or related to the malignant state, including not only tumor growth but also invasion and metastasis. We will use the human breast cancer cell line MCF-7 for these studies. MCF-7 requires estrogen in order to form tumors in nude mice but is not dependent on estrogen for growth in culture. Clones of MCF-7 differing in their properties will be compared for tumorigenicity, properties in culture associated with malignancy and the effect of hormones on both.